Since the successful development of high-activity catalysts for polyethylene in 1970's, there has been rapid progress with respect to the researches into the catalysts for ethylene polymerization and three systems in terms of the composition of the catalysts, i.e. chromium type high-activity catalysts, Ziegler-Natta type high-activity catalysts and metallocene type high-activity catalysts, have been formed. Among them, Ziegler-Natta type high-activity catalysts are very important in the field of catalysts for their excellent performance and the processes for preparing them have been developed from the grinding process as well as the suspending-impregnating process utilized in the early stages to the chemical reaction process utilized at present. Processes for producing polyethylenes are continuously renewed and among them, the gas phase process for synthesizing polyethylenes are being widely utilized for low investment to equipment, low energy consumption, excellent product performance and high economical benefits. Recently, exploitation of various novel catalysts makes the gas phase process develop rapidly, and there are many patents issued with respect to the processes for preparing such catalysts.
For example, U.S. Pat. No. 4,497,904 discloses a catalyst which has a narrow particle size distribution and is very suitable for the gas phase polymerization process, prepared by reacting butyloctyl magnesium as raw material with butyl chloride to form a spherical magnesium chloride as a carrier of the catalyst, and then reacting a halide containing titanium with said carrier. The catalyst is disadvantageous in that dialkyl magnesium as raw material is expensive, which results in high cost. It is described in U.S. Pat. No. 4,438,019 that powdered magnesium, titanium tetrachloride, propyl titanate and butyl chloride are reacted to obtain a catalyst for polymerizing olefins; though said catalyst has high initial activity, it gives rise to the appearance of hot spots during the polymerization and the formation of agglomerates of molten polyethylene, or fragmentation of the catalyst particles; moreover, said catalyst must be oxidized from Ti.sup.3+ to Ti.sup.4+ prior to use. To overcome the disadvantage, U.S. Pat. No. 4,894,424 describes that an appropriate amount of water is added during the preparation of the catalyst to inactivate it, reduce the initial polymerization reaction rate and prevent the appearance of hot spots. However, the preparation of such a catalyst is too complicated and the amount of water to be added should be strictly controlled so as to inactivate the catalyst.
U.S. Pat. Nos. 4,349,648 and 4,302,565 disclose catalysts for gas phase polymerizing ethylene, prepared by reacting anhydrous magnesium chloride with tetrahydrofluran and titanium tetrachloride to form a complex, then impregnating said complex onto silica gel as carrier, followed by drying the resulting product to form a main catalyst component having good flowability and finally activating said component with an alkyl aluminum compound. Such catalysts has very high initial activity, giving rise to the appearance of hot spots and the formation of agglomerates of polyethylene, and the resulting polyethylenes have low and medium densities (from 0.91 to 0.94 g/cm.sup.3).
In order to overcome the above disadvantages, the present inventors proposed a catalyst in Chinese Patent Application No. 93112345.3, which is prepared by using powdered magnesium as raw material to form a complexed magnesium halide carrier having reducing capability and then supporting active components onto said carrier by impregnating, with more than 80% of titanium being trivalent. Said catalyst is advantageous for a simple preparing procedure, low initial activity of polymerization reaction, smooth and stable reaction, high catalytic activity, good morphology of the resulting polymer particles and lack of fine particles. However, said catalyst must be subjected to prepolymerization so as to be readily fed into the fluidized bed and thus there is a room for improvement.